[TxGator]

Take a look at the yamaha website. Its some times hard to draw a conclusion, but by comparing the closest boats possible i came up with this. The old model Nautic Star, the 1900 weighs about 150#s more than the current 1910 model. It is also appx 10" narrower.
1900 w/4s 150 ............1910 w/4s 115
5.37mpg@ 22mph .........5.2mpg @ 20mph
5.2 @ 34 ...........................4.6 @ 35
4.39 @ 44 .........................3.9 @ 43wot
3.4 @ 53.6wot

If I were going to run around at the boats peak point of effiiciency it really wouldnt make a difference. But like most I want to be going at least 35 - 40 mph. These numbers make me think that the boat will be most economical with the larger engine. In actuality, it will probably be a bit better than these numbers since the 1910s weigh less and is narrower.

It may not play out for all boats but for this one it did. I would get the max size engine.

Gator